But more than both of these considerations put together and multiplied by a hundred, if you lose your teeth, you can no longer grind your food well; and then comes indigestion with its train of horrors. How may the teeth be preserved? Simply, by keeping them clean! A clean tooth cannot decay. You may eat sweet things, acids, take hot drinks, ice creams,--you may abuse your teeth in a hundred ways,--if you will keep them clean, they will not decay. I will show you as many white blackbirds, as you will show me clean white teeth beginning to decay. How shall they be kept clean? I answer with a tooth-pick, used thoroughly after eating, and followed by rinsing the mouth, and the morning and evening use of a tooth-brush with a powder composed of pulverized soap and prepared chalk. In addition to this, cultivate the habit of sleeping with your mouth shut. That dryness and bad taste in the mouth which come of sleeping with it open, is always injurious to everything within the mouth, including the teeth. And, perhaps, this is the best place to speak of the error or misfortune of sleeping with the mouth open, in its influence upon the respiratory apparatus. I cannot agree with the famous Catlin, who attributes so much to this bad habit. But really it is difficult to read his remarkable little work, without being convinced that sleeping with the mouth open is a most unfortunate habit. The most obvious mischief is the introduction through the open mouth and wind-pipe of dust and other minute objects, which the nose would strain out. The opening in the nose through which the air must pass, is only a narrow fissure, and its sides are armed with numerous hairs, which reach over and intertwine with those of the opposite wall, thus making it very difficult for particles of dust to pass through into the lungs. This point in Mr. Catlin's argument is too obviously true to need any special proofs; and perhaps another point of less moment is sufficiently obvious; viz., if the air be allowed to pass directly through the wide-open mouth into the lungs, its temperature when permeating the lung tissue is too low, and thus injury to that delicate tissue results; but if the air passes through the tortuous and contracted nasal passages, it is brought into such immediate contact with the blood in the lining membrane of those passages, that it is modified, and the lungs themselves are saved from the rude shock of a raw cold breath. I have now given the more patent of the reasons for keeping the mouth shut while sleeping, and will only add that the habit of sleeping with the mouth shut, may be formed by a careful clearing of the nasal passages on lying down, and by going to sleep with a determination to keep the lips closed. Observing these rules, and being careful not to sleep with the head too low, you will soon awaken in the morning with the lips closed, and with the mouth moist and sweet. VENTILATION.